Liquid crystal display elements which utilize optical anisotropy and dielectric anisotropy of liquid crystal materials have been used in watches, electric computers, word processors, and television sets etc, and consumption thereof has a tendency to increase year by year. A liquid crystal phase is positioned between a solid phase and a liquid phase, and classified roughly into a nematic phase, a smectic phase and a cholesteric phase. Amongst them, display elements utilizing nematic phases are most widely used. On one hand, although many display modes have been planned hitherto, three modes such as TN display mode, STN display mode and TFT display mode are now main current. Further, as driving modes, a static driving mode, a time-division driving mode, an active matrix driving mode and two-frequency driving mode etc. are known.
Recently, display elements are required to have higher display properties, and thus there have been increased needs for display elements of active matrix driving modes, typically TFT display mode. Liquid crystal materials used in any type of display elements should be stable to external environment factors such as moisture, air, heat and light, express liquid crystal phase in a temperature range as wide as possible which centering around the room temperature, and have low viscosity, low driving voltage, large .DELTA. .epsilon. and optimum .DELTA.n. There is, however, no compound presently which satisfies all these conditions as a single compound, and thus liquid crystal compositions obtained by mixing several liquid crystalline compound or non-liquid crystalline compounds are used now.
One of the characteristics particularly required for liquid crystal display elements of TFT display mode is that display plane has high contrast. Therefore the liquid crystal material used for this object is required to have high specific resistant value, i.e. high voltage holding ratio (V.H.R.) in addition to the above-mentioned conditions. Further, liquid crystal display elements of TFT display mode are required to have low voltage driving, and liquid crystal compositions having higher .DELTA. .epsilon. than conventionally used liquid crystal materials are necessary to satisfy the needs.
In liquid crystalline compounds generally known, those having cyano group are dominant, for which there are problems such as drastical lowering of voltage holding ratio at a high temperature because of large temperature dependency of voltage holding ratio, in the case of using such compounds in liquid crystal display elements of TFT display mode. Therefore, liquid crystalline compounds containing cyano group are not generally used in liquid crystal display elements of TFT display mode, in spite of having high .DELTA. .epsilon.. In order to improve it, liquid crystal materials having high .DELTA. .epsilon. while showing high specific resistant value have been developed. As liquid crystalline compounds having high specific resistant values, fluorine type compounds are suitable. These compounds, in general liquid crystalline compounds having (a) fluorine atom(s) as (a) substituent(s), are known as described below. For example, compounds expressed by the following formula (10) are disclosed in Japanese Patent Publication Hei 02-40048. ##STR1##
The said compounds (10) have been used industrially, because of having higher specific resistant values than compounds with cyano group, but they cannot realize sufficient low-voltage driving because of low .DELTA. .epsilon. such as about 4.
As compounds having higher .DELTA. .epsilon. than the above-mentioned compounds (10), compounds having trifluorophenyl group as partial structure expressed by the following formula (11) are disclosed in Japanese Patent Application Laid-open Hei 02-233626. ##STR2##
The said compounds, however, have not sufficient .DELTA. .epsilon. such as about 8, and also have more fluorine atom(s) introduced than the above-mentioned compounds of the formula (10) and thus have narrower temperature range of liquid crystal phase than (10), so that they are not suitable for use as one component of liquid crystal compositions. Further, considering a clear point, 1-(trans-4-(trans-4-propylcyclohexyl)cyclohexyl)-2-(3,4,5-trifluorophenyl) ethane (11) has a point about 60.degree. C. lower than the corresponding monofluoro compound, that is, 1-(trans-4-(trans-4-propylcyclohexyl)cyclohexyl)-2-(4-fluorophenyl)ethane, and about 25.degree. C. lower than the corresponding difluoro compound, that is, 1-(trans-4-(trans-4-propylcyclohexyl)cyclohexyl)-2-(3,4-difluorophenyl)eth ane(10), respectively.
Further, compounds expressed by the formulae (12) and (13) are disclosed in Japanese-translated PCT Patent Applications Laid-open Hei 5-501735 and 2-501311. ##STR3## These compounds have relatively high .DELTA. .epsilon. (for example .DELTA. .epsilon. of the compound (13) being about 7), but their compatibility at a low temperature with the existing liquid crystalline compounds is very poor, so that they are not suitable for a constituting component of a liquid crystal composition. In order to improve the said compatibility, compounds having (a) fluorine atom(s) introduced in R, i.e. an alkyl group, as disclosed in Japanese-translated PCT Patent Application Laid-open Hei 04-506817. The disclosed compound is 2- or 3-cyclic compound having terminal cyclohexyl group and phenyl group, such as the compound expressed by the following formula (14). Derivatives having covalent bonds as bonding groups are only disclosed and there is not disclosed any derivative with other bonding group such as 1,2-ethylene group. Further, (a) substituent(s) on the above-mentioned terminal phenyl group is(are) limited to (a) fluorine atom(s), and there is no description about other substituents such as fluoroalkyl group or fluoroalkoxy group, and also their presences cannot be read from the specification. ##STR4##
Further, such compounds have not improve compatibility, particularly the compound of the formula (14) does not express any liquid crystal phase. Therefore, a compound having high .DELTA. .epsilon. and good compatibility has not being known yet, and in the case of using the liquid crystalline compound with high .DELTA. .epsilon. as a composition component, the mixing ratio cannot be increased and the magnitude in .DELTA. .epsilon. of the composition may be limited under the present conditions. Therefore, a liquid crystalline compound having excellent compatibility with conventional liquid crystalline compounds has been waited.
An object of the present invention is to solve the above-mentioned disadvantages of the prior arts and provide a novel liquid crystalline compound which has a wide temperature range of liquid crystals, low threshold voltage, high stability and superior compatibility with other liquid crystal compounds, a liquid crystal composition containing the said compound, as well as a liquid crystal display element by use of the said composition.